The present invention relates to variable displacement compressors employed in automotive air conditioning systems.
A typical variable displacement compressor has a housing that houses a crank chamber and supports a rotatable driving shaft. Cylinder bores extend through a cylinder block, which forms part of the housing. A piston is accommodated in each cylinder bore. A swash plate is supported to rotate integrally with the drive shaft, while inclining in the axial direction. Rotation of the swash plate reciprocates each piston and draws refrigerant gas into the associated cylinder bore, compresses the refrigerant gas, and disharges the compressed refrigerant gas into a discharge chamber. A displacement control valve adjusts the difference between the pressure of the cylinder bores and the pressure of the crank chamber (first differential pressure .DELTA.P1) to alter the inclination of the swash plate with respect to a plane perpendicular to the drive shaft. The stroke of the pistons is changed in accordance with the inclination of the swash plate to vary the displacement of the compressor.
Typically, the variable displacement compressor is connected to an automotive engine by an electromagnetic clutch. The clutch is actuated to connect the engine to the compressor when activating the air conditioning system.
When the inclination of the swash plate is large, that is, when the displacement of the compressor is large, an increase in the engine speed may rotate the drive shaft at a high speed. In such case, the compression load increases in a sudden manner. This increases the product of the pressure between contacting surfaces of moving parts and the velocity of the contacting moving parts (i.e., Pv value). As a result, the life of the moving parts and the compressor is shortened.
Such shortcomings have been overcome by de-actuating the electromagnetic clutch to stop operation of the compressor in accordance with parameters indicating acceleration of the automobile. For example, operation of the compressor is stopped when the engine speed increases, or when the detected engine intake air pressure and acceleration pedal depression exceed predetermined values. However, such de-actuation of the compressor increases fluctuations in the temperature of the air blown through an evaporator, which is connected to the compressor by way of an external refrigerant circuit. As a result, warm air enters the passenger compartment and makes the passenger compartment uncomfortable. Additionally, the shifting of the electromagnetic clutch between actuated and de-actuated states produces shocks.
Compressors that continue operation during acceleration of the vehicle are also known. However, such compressors interfere with acceleration and lower fuel consumption.
Accordingly, U.S. Pat. No. 4,872,814 proposes a variable displacement compressor that overcomes these shortcomings. The compressor has a displacement shifting mechanism that shifts displacement from a maximum state toward a minimum state when the rotating speed becomes too high. As shown in FIG. 18, the displacement shifting mechanism includes a pressurizing passage 101 that connects a crank chamber with a discharge chamber (neither shown). A valve body 102 is attached to a drive shaft 103 by means of springs 105, 106 to rotate integrally with the drive shaft 103. The pressurizing passage 101 has a port 104. As shown by the chain lines in FIG. 18, the valve body 102 moves relative to the drive shaft 103 in a direction parallel to the axis L of the drive shaft 103 and in a direction perpendicular to the axis L. Movement of the valve body 102 in these two directions opens and closes the port 104 to the valve body 102. Under normal conditions, the forces of the springs 105, 106 cause the valve body 102 to close the port 104. The valve body 102 is arranged in a crank pressure region 101a, which is located downstream of the port 104 in the pressurizing passage 101.
The valve body 102 includes a weight 102a. When the displacement of the compressor is large, if the engine speed N increases and causes the rotating speed of the drive shaft 103 to exceed a predetermined limit value Nc, which is shown in FIG. 19, the centrifugal force applied to the weight 102a moves the valve body 102 against the force of the spring 105 in a direction perpendicular to axis L and opens the port 104. When the port 104 is opened, the refrigerant gas in the discharge chamber enters the crank chamber through the pressurizing passage 101 and increases the pressure of the crank chamber. Consequently, the first differential pressure .DELTA.P1 increases and decreases the displacement of the compressor. This decreases compression load and avoids excessive friction of the moving parts.
If the condenser becomes too warm when the displacement of the compressor is large, for example, when cooling of the condenser becomes insufficient, the pressure of the discharge chamber becomes abnormally high. In such case, if the difference between the pressure of the discharge chamber and the pressure of the crank pressure region 101 (second differential pressure .DELTA.P2) exceeds a predetermined limit value .DELTA.Pc, the discharge pressure communicated through the port 104 moves the valve body 102 toward the drive shaft 103 against the pressure of the crank pressure region and the force of the spring 106 to open the port 104. Thus, the refrigerant gas in the discharge chamber enters the crank chamber through the pressurizing passage 101 and increases the pressure of the crank chamber. This decreases the displacement of the compressor. As a result, the compression load decreases and reduces friction in moving parts.
The refrigerant gas in the discharge chamber is drawn into the crank chamber to increase the pressure of the crank chamber and decrease the displacement of the compressor when the rotating speed N of the drive shaft 103 exceeds a predetermined limit value Nc or when the second differential pressure .DELTA.P2 exceeds the predetermined limit value .DELTA.Pc.
However, this compressor has the shortcomings described below.
(1) The shifting of the displacement from a maximum state toward a minimum state improves the acceleration performance of the vehicle and fuel efficiency. However, there is a large displacement difference between the maximum displacement and the minimum displacement. For example, if the displacement is 100% in the maximum displacement state, the displacement is 1% to 10% in the minimum displacement state. Therefore, a relatively long time is necessary to return the compressor to the maximum displacement state from the minimum displacement state. This results in insufficient cooling of the passenger compartment. Furthermore, shifting of the displacement from the maximum state to the minimum state and then back to the maximum state causes fluctuations in the torque applied to the engine. This may lower the driving performance of the vehicle.
(2) The valve body attached to the drive shaft 103 moves when receiving centrifugal force from the weight 102a. This unbalances the drive shaft 103, which produces vibration and torque fluctuation.
(3) When the compressor displacement is large but the rotating speed N of the drive shaft 103 and the second differential pressure .DELTA.P2 are both below their limit values Nc, .DELTA.Pc, the displacement is not decreased. In FIG. 19, the cross-hatched zone S represents a range in which the rotating speed N of the drive shaft 103 and the second differential pressure .DELTA.P2 are both close to but below their limit values Nc, .DELTA.Pc. To stop operation of the compressor in the cross-hatched zone S and prevent undesirable wear of the moving parts, the limit values Nc, .DELTA.Pc must both be lowered to Nc', .DELTA.Pc', respectively, as shown in FIG. 19. The load applied to the compressor is excessive when the rotating speed N and the second differential pressure .DELTA.P2 are in the cross-hatched zone S. On the other hand, if the rotating speed N or the second differential pressure .DELTA.P2 were to exceed the associated lower limit value Nc', .DELTA.Pc' without entering the cross-hatched zone S, the friction load would be acceptable. Nevertheless, the displacement would be decreased. In other words, the displacement would be unnecessarily decreased, which would interfere with the cooling process.